In a projector which projects light in the R wavelength range, G wavelength range, and B wavelength range for displaying a color image on a screen, it is necessary to project the light beams in the respective wavelength ranges onto one point so as to form the color image. As a configuration for projecting light beams in the respective wavelength ranges onto one point, there is known a technique which scans light beams in the respective wavelength ranges by a mirror mechanism employing MEMS (Micro Electro Mechanical Systems).
This configuration has following problems; when the mirror scans the light beams in the respective wave lengths, integrated in the horizontal direction, the timings when the light beams in the respective wavelength ranges are projected are different from one another, and scan-angle ranges of the respective wavelength ranges are various.
As a prior art for solving the problems above, there is known a technique described in the Patent Document 1. The Patent Document 1 describes that the RGB light beams are combined by using a beam combiner which is made up of a combination of four rectangular prisms referred to as “x-cube”, or a dichroic prism such as a beam combiner made up of three optical portions with which problems held by the x-cube are solved. In addition, it is also known that a typically used projector employs a cross prism to combine the RGB light beams.
By way of example, according to the Patent Documents 2 and 3, there is known a technique which employs a prism to combine laser beams outputted from a semiconductor laser.
The Patent Documents 4 and 5, for instance, suggest a device that employs a directional coupler to perform optical multiplexing and branching in optical communication. The Patent Documents 4 and 5 disclose a technique that uses a waveguide to constitute the directional coupler for multiplexing or branching two wavelengths.